Update mailing lists

Signed-off-by: Fabian Reinartz <freinartz@google.com>

content/community.html

@@ -14,8 +14,11 @@ title: Community
       (for the easiest start, <a href="https://riot.im/app/#/room/#freenode_#prometheus:matrix.org">join via Riot</a>)
     </p>
     <p>
-      <strong>Usage mailing list:</strong>
-      <a href="https://groups.google.com/forum/#!forum/prometheus-users">prometheus-users</a> – for discussions around Prometheus usage and community support
+      <strong>User mailing lists:</strong>
+      <ul>
+        <li><a href="https://groups.google.com/forum/#!forum/prometheus-announce">prometheus-announce</a> – for announcements like new releases</li>
+        <li><a href="https://groups.google.com/forum/#!forum/prometheus-users">prometheus-users</a> – for discussions around Prometheus usage and community support</li>
+      </ul>
     </p>
     <p>
       <strong>Twitter:</strong>